Known position initialization techniques have been used since the first RTK products were sold. In a prior-art method Allison, M. T. et al., 1994, Determination of phase ambiguities in satellite ranges, U.S. Pat. No. 5,359,332, Issued October 25, the distance and orientation of a rover antenna relative to a reference antenna is used in the estimation of the carrier phase ambiguities on GNSS signals for the purposes of centimeter level positioning. An apparatus is also described in which an azimuth measuring device is coupled with a fixed distance rover antenna mount. The method and apparatus described focuses on single reference (base) RTK positioning and does not address the estimation process needed for GVRS positioning.
Known Position Input
Introduction
FIG. 12 schematically illustrates a scenario using a GNSS rover with correction data for point surveying. A user 1100 has a rover receiver (rover) 1105 which is mounted on a range pole 1110 or in some cases is a hand-held, or machine-mounted unit without a range pole. Rover 1105 includes a GNSS antenna 1115 and a communications antenna 1120. Rover 1105 receives at its GNSS antenna 1115 the signals from GNSS satellites 1125, 1130, 1135, 1140, 1145, etc. Rover 1105 also receives at its communications antenna 1120 correction data from a corrections source 1150 via a communications link 1155. The communications link is, for example, a radio link or mobile telephone link, or any other suitable means of conveying the correction data to the rover 1105. The correction data can be of any suitable type for improving the positioning accuracy of rover 1105, such as: differential base station data from a base station serving as corrections source 1150, or virtual reference station data from a network of reference stations serving as corrections source 1150 (WAAS is one example), or precise orbits and clocks data from a GVRS service such as that described in U.S. Provisional Application Patent No. 61/277,184 filed 19 Sep. 2009. In the example of FIG. 12, the phase center of GNSS antenna 1115 is determined and reduced for the height and orientation of the range pole 1110 to the survey point 1160. The position of the rover can be determined at each measurement epoch while static or kinematic. In this example, the location of point 1160 is determined via static occupation, followed by a segment of kinematic positioning, then another static occupation of point 1170. The location of occupied points is often saved as part of a measurement process.
There are several applications that can take advantage of knowledge of the user location in order to reduce solution convergence times. For example, machine control operators from time-to-time need to stop and shut down their machine during meal times or overnight. The position of the machine is therefore often accurately known prior to the tracking interruption. Similarly, a surveyor often measures the location of points of interest via static occupation, separated by periods of kinematic positioning. If tracking is interrupted while moving, the surveyor can return to a previously surveyed mark (like point 1160 or point 1170 in FIG. 12) and restart operation. The use of known position information can accelerate re-convergence of the estimation process.
The Known Position Input scheme presented below is applicable to GVRS rover processing, but it can also be used for single-base, VRS, and all RTK techniques.
Geometry Filter Seeding
The known rover position information can be used to seed the (X,Y,Z) position states of the Geometry Filter via tightly constrained position covariance terms. The known position in this case would help to accelerate the convergence of all states in the Geometry Filter and therefore the overall Float filter and iFlex solution.
The disadvantage of position seeding is that if the input coordinates are incorrect, this will corrupt the Geometry filter into the future, unless the filter is reset.
Known Position Input Via Auxiliary Code-Carrier Filter Aiding
The auxiliary code-carrier filter results can be used to provide a means of inputting position information to the FAMCAR process. An advantage of this approach is that the position aiding process does not alter the underlying filters, but rather is just applied to the output of the Auxiliary Code-Carrier Filters. The position aiding process is analogous to that used for iono-bridging. In the case of position aiding, the Auxiliary Code-Carrier filter results are modified, whereas for iono-bridging, the ionospheric filter results are modified.
The Auxiliary Code-Carrier filter bank normally produces iono-free ambiguity estimates for each tracked satellite based on iono-free code measurements. Iono-free code is inherently noisy and therefore the iono-free ambiguity estimates of the Auxiliary Code-Carrier filters are also noisy and only contribute a small amount of information to the float solution.
When the position of the rover (A) is known, the geometric range from rover to each satellite is given by:ρAi√{square root over ((xi−XA)2+(yi−YA)2+(zi−ZA)2)}  (14)where:    (xi, yi, zi) Cartesian WGS84 coordinates of satellite i, given by the satellite ephemeris,    (XA, YA, ZA) Cartesian WGS84 coordinates of the rover (known position).
The reference receiver (R) location is also known and the geometric range from reference receiver to each satellite is given as:ρRi√{square root over ((xi−XR)2+(yi−YR)2+(zi−ZR)2)}  (15)where:    (XR,YR,ZR) Cartesian WGS84 coordinates of the reference receiver. Note that in the case of GVRS processing, the reference receiver is synthetic, nevertheless the reference receiver coordinates are defined.
The single difference iono-free carrier phase ambiguities are estimated for each satellite via the following equation (with all quantities given in meters):ΔifNRAi=ΔifΦRAi└ΔρRAi+ΔτRAi+ΔκRA┘  (16)where:    ΔifNRAi single-difference iono-free carrier phase ambiguity for satellite i,    ΔifΦRAi single-difference iono-free carrier phase observation for satellite i,    ΔρRAi single-difference geometric range for satellite i (ΔρRAi=ρAiρRi),    ΔτRAi single-difference tropospheric bias for satellite i, based on a tropospheric model (e.g. Hopfield, Goad-Goodman, etc), or based on the estimated tropospheric bias parameters from the Geometry Filter,    ΔκRA single-difference receiver clock bias (=reference receiver clock bias minus the rover receiver clock bias).
The uncertainty of the rover location is expressed in terms of the following position covariance matrix:
                              Q          p                =                  [                                                                      q                  xx                                                                              q                  xy                                                                              q                  yz                                                                                                      q                  yx                                                                              q                  yy                                                                              q                  yz                                                                                                      q                  zx                                                                              q                  zy                                                                              q                  zz                                                              ]                                    (        17        )            where qxx refers to the variance of the x-coordinate, qxy, refers to the covariance of the x and y coordinates etc.
The variance of the rover-satellite geometric range is obtained by projecting the rover position covariance matrix into the direction of the satellite according to:
                              σ                      ρ            A            i                    2                =                                            [                                                                                          a                      x                      i                                                                                                  a                      y                      i                                                                                                  a                      z                      i                                                                                  ]                        ⁡                          [                                                                                          q                      xx                                                                                                  q                      xy                                                                                                  q                      xz                                                                                                                                  q                      yx                                                                                                  q                      yy                                                                                                  q                      yz                                                                                                                                  q                      zx                                                                                                  q                      zy                                                                                                  q                      zz                                                                                  ]                                ⁡                      [                                                                                a                    x                    i                                                                                                                    a                    y                    i                                                                                                                    a                    z                    i                                                                        ]                                              (        18        )            where:
      a    x    i    =            -              (                              x            i                    -                      X            A                          )                    ρ      A      i           Partial derivative of the rover-satellite geometric range with respect to the rover XA coordinate;
      a    y    i    =            -              (                              x            i                    -                      Y            A                          )                    ρ      A      i           Partial derivative of the rover-satellite geometric range with respect to the rover YA coordinate;
      a    z    i    =            -              (                              z            i                    -                      Z            A                          )                    ρ      A      i           Partial derivative of the rover-satellite geometric range with respect to the rover ZA coordinate.
The variance of the computed single difference iono-free carrier phase ambiguity is computed by applying the law of propagation of variances to (16):σΔifNRAi2=σΔifΦRAi2+σΔρRAi2+σΔτRAi2+σΔκRAi2  (19)where:    σΔifNRAi2 variance of the single-difference iono-free carrier phase ambiguity for satellite i;    σΔifΦRAi2 variance of the single-difference iono-free carrier phase measurement for satellite i;    σΔρRAi2 variance of the single-difference geometric range term for satellite i;    σΔτRAi2 variance of the single-difference tropospheric model value for satellite i;    σΔκRAi2 variance of the single-difference receiver clock bias term.
Normally the dominant errors in (19) relate to the geometric-range term σΔρRAi2, and the carrier phase measurement σΔifΦRAi2; the other error sources are often ignored.
FIG. 13 illustrates the use of known position information in the updating of the auxiliary code-carrier filter results. This flowchart is an expansion of steps 815 and 820 in FIG. 7.
The position aiding process is terminated as soon as the geometry filter has sufficiently converged. The geometry filter convergence test is conducted at 1305. If known position is available (1310), then at 1320 ionospheric-free carrier phase ambiguities are computed based on the known position input. The ionospheric-free carrier phase ambiguities are stored to the Geometry Cache as part of step 1320.
It is important to monitor cycle slips in the multi-frequency bands to ensure that the ionospheric-free carrier phase ambiguities stored in the Geometry cache are consistent with the current phase (1325). The Auxiliary Code-Carrier Filter ambiguity results are updated with the Geometry Cached ambiguities in step 1330. The results of the Auxiliary Code-Carrier Filters are posted at 1335, and used in the FAMCAR combination step 820.
FIG. 14 schematically describes the FAMCAR filtering process with position aiding. FIG. 14 is derived from the standard FAMCAR filtering process shown in FIG. 5. The known position processor, 1405, accepts single-differenced ionospheric-free phase data from 605/610, plus the ionospheric-free phase ambiguities 645 produced by the Auxiliary Code-Carrier Filter bank 625. When known position information 1402 is provided to the known position processor, it produces ionospheric-free phase ambiguities that replace those produced by the Auxiliary Code-Carrier Filter bank (1410). The known position aided ambiguities are used in the FAMCAR combination step 655. Finally, the position and float ambiguity estimates are reported at step 660.
Known Position Input Via Code-Carrier Filters
The low accuracy of the Auxiliary Code-Carrier Filter ambiguity results normally means that they don't contribute significantly to the final FAMCAR combined float solution. This makes the Auxiliary Code-Carrier Filter results well suited to use for known position input. Furthermore, the iono-free carrier phase combination used in the Auxiliary Code-Carrier Filters, means that the known position range computations can be formed without being impacted by ionospheric bias. Ionospheric bias is a significant error source for GVRS processing.
The known position input could also be handled by modifying the Code-Carrier Filter results, in a manner which is analogous to that used in the Auxiliary Code-Carrier Filter results. The disadvantage of this approach is that the Code-Carrier Filters nominally use the wide-lane carrier phase combination, which contains an ionospheric bias. Second, the code-carrier filter results contribute significantly to the FAMCAR combined float solution therefore this information would be compromised if replaced by the known position input.
Rather than replacing the Auxiliary Code-Carrier Filter results with known position information, an alternative is to generate a parallel bank of Auxiliary Code-Carrier Filter results devoted to known position information input.
Termination of the Known Position Aiding Process
The known position aiding process outlined does not corrupt any FAMCAR component filter, just the uncombined filter results (output) are modified prior to the FAMCAR filter combination step. The known position aiding process is automatically terminated when:                The known position is deemed to be incorrect;        The Geometry filter has converged sufficiently that the known position aiding is no longer required.        
The known position information is deemed suspect or incorrect if the float/iFlex solution fails a statistical test of the mean and/or variance.
When the position states of the Geometry filter have converged, i.e. all variances <for example 0.002 m2. In this case, the position aiding no longer adds significant information to the float solution.
Performance of Known Position Input Processing
As illustrated in FIG. 3, convergence and re-convergence of the horizontal position accuracy can take 15-20 minutes to achieve centimeter-level. With known position input, the convergence time is normally reduced to a few seconds up to for example 1 minute.
Following is a summary of some of the inventive concepts described herein:
[Iono Bridging]                1. A positioning method, comprising                    a. Obtaining GNSS data derived from multi-frequency signals received at a rover antenna,            b. Obtaining correction data derived from a network of reference stations,            c. At each of a plurality of epochs, using the GNSS data and the correction data to estimate values defining a rover antenna position and a set of multi-frequency ambiguities,            d. Using an ionospheric filter to model variation in ionospheric bias per satellite,            e. Estimating a set of ionospheric carrier-phase ambiguities at least when the multi-frequency ambiguities have attained a predetermined precision,            f. Caching the estimated ionospheric carrier-phase ambiguities,            g. Detecting an interruption of signal received at the rover antenna,            h. Determining reacquisition of signals received at the rover antenna,            i. Predicting an ionospheric bias per satellite over an interruption interval,            j. For each satellite, combining a cached ionospheric carrier-phase ambiguity with a predicted ionospheric bias to obtain a post-interruption ionospheric ambiguity estimate,            k. Using the post-interruption ionospheric ambiguity estimates to aid estimation of at least a rover antenna position subsequent to the reacquisition.                        2. The method of 1, wherein aiding comprises, at each of a plurality of epochs after reacquisition of signals, using the GNSS data and the correction data and the post-interruption iono ambiguity estimates to estimate values defining an aided rover antenna position and an aided set of multi-frequency ambiguities.        3. The method of 2, further comprising determining a precision of the post-interruption rover antenna position and of each of the multi-frequency ambiguities.        4. The method of 3, wherein when the precisions of the post-interruption rover antenna position and of each of the multi-frequency ambiguities has achieved a predetermined threshold, using the post-interruption ionospheric ambiguity estimates to aid estimation is terminated.        5. The method of one of 1-4, wherein rover-satellite ionospheric biases are substantially uncorrelated with reference station-satellite ionospheric biases.        6. The method of one of 1-5, wherein the ionospheric filter models an ionospheric bias per satellite which is single-differenced between rover data and correction data.        7. The method of one of 1-6, wherein the set of ionospheric carrier-phase ambiguities comprises a single-differenced ionospheric ambiguity per satellite        8. The method of one of 1-7, wherein detecting an interruption comprises determining that fewer than four satellites are continuously observed over a predetermined interval.        9. The method of one of 1-8, wherein determining reacquisition comprises determining that at least four satellites are continuously observed over a predetermined interval.        10. The method of one of 1-9, wherein the estimated rover antenna position has a precision which is better than a precision that would be obtained without use of the generated set of ionospheric ambiguity estimates.        11. The method of one of 1-10, further comprising terminating using the post-interruption ionospheric ambiguity estimates to aid estimation when substantially no further benefit is obtained therefrom.        12. The method of one of 1-11, wherein the values defining the rover antenna position are estimated in a filter, the method further comprising monitoring precisions of the values estimated and terminating using the post-interruption ionospheric ambiguity estimates to aid estimation of rover antenna position when a precision threshold is achieved for values defining the rover position.        13. The method of one of 10-11 wherein terminating using the post-interruption ionospheric ambiguity estimates to aid estimation of rover antenna position comprises continuing to use the GNSS data and the correction data to estimate values defining a rover antenna position.        14. The method of one of 1-13, further comprising predicting tropospheric bias and using predicted tropospheric bias to aid the estimation of at least a rover antenna position subsequent to the reacquisition.        15. The method of one of 1-14, wherein the set of ionospheric carrier-phase ambiguities is determined as a weighted average of integer ambiguity candidate sets.        16. The method one of 1-15, wherein caching of the estimated ionospheric phase ambiguities is deferred until satellite tracking is determined to be stable and within predetermined parameters.        17. The method of one of 1-16, further comprising estimating time-wise variation of an ionospheric bias per satellite.        18. The method of one of 1-17, further comprising estimating a change in ionospheric carrier-phase ambiguities after detecting an interruption of signal received at the rover antenna.        19. The method of one of 1-18, wherein using the post-interruption ionospheric ambiguity estimates to aid estimation of at least a rover antenna position subsequent to the reacquisition comprises combining the post-interruption ionospheric ambiguity estimates with estimates of other parameters from a set of factorized filters.        20. The method of 19, wherein the post-interruption ionospheric ambiguity estimates are substituted for estimates from a bank of ionospheric filters.        21. Apparatus for performing a method according to one of 1-20.        22. A computer program comprising instructions for causing an apparatus to perform a method according to one of 1-20.        23. A computer program product comprising a tangible computer-readable medium embodying instructions for causing an apparatus to perform a method according to one of 1-20.        
[Known Position]                1. A positioning method, comprising                    a. Obtaining GNSS data derived from multi-frequency signals received at a rover antenna,            b. Obtaining correction data derived from a network of reference stations,            c. At each of a plurality of epochs, using the GNSS data and the correction data to estimate values defining a rover antenna position and a set of multi-frequency ambiguities,            d. Estimating an ionospheric-free carrier-phase ambiguity per satellite based on a known rover antenna position, and            e. Using the estimated ionospheric-free carrier-phase ambiguities to assist in determining an aided rover antenna position.                        2. The method of 1, wherein using the estimated ionospheric-free carrier-phase ambiguities to assist in determining an aided rover antenna position comprises combining the estimated ionospheric-free carrier-phase ambiguity with an estimated widelane ambiguity and with an estimated ionospheric-free ambiguity and with values defining the known rover antenna position to obtain values defining an aided rover antenna position and aided multi-frequency ambiguities.        3. The method of one of 1-2, further comprising terminating using the estimated ionospheric-free carrier-phase ambiguities to assist in determining an aided rover antenna position when substantially no further benefit is obtained therefrom.        4. The method of one of 1-3, further comprising monitoring precision of an unaided rover antenna position estimate to determine when substantially no further benefit is obtained from using the estimated ionospheric-free carrier-phase ambiguities to assist in determining an aided rover antenna position.        5. The method of one of 2-4, wherein the widelane ambiguities are estimated in a set of code-carrier filters.        6. The method of one of 2-5, wherein the ionospheric-free ambiguities are estimated in a geometry filter.        7. The method of one of 1-6, wherein the ionospheric-free carrier-phase ambiguity per satellite based on a known rover antenna position is computed using the known rover antenna position and observed carrier-phase measurements.        8. The method of one of 1-7, wherein the ionospheric-free carrier-phase ambiguity for at least one satellite based on a known rover antenna position is improved using a prevailing tropospheric bias on the respective satellite.        9. The method of one of 1-8, wherein using the estimated ionospheric-free carrier-phase ambiguities to assist in determining an aided rover antenna position estimated ionospheric-free carrier-phase ambiguity estimates with estimates of other parameters from a set of factorized filters.        10. The method of 9, wherein the estimated ionospheric-free carrier-phase ambiguity estimates are substituted for estimates from a bank of auxiliary code-carrier filters.        11. The method of 10, further comprising creating a separate bank of auxiliary code carrier filter results for the known position results so that normal auxiliary code carrier filter results remain unaffected.        12. Apparatus for performing a method according to one of 1-11.        13. A computer program comprising instructions for causing an apparatus to perform a method according to one of 1-11.        14. A computer program product comprising a tangible computer-readable medium embodying instructions for causing an apparatus to perform a method according to one of 1-11.        The foregoing description of embodiments is not intended as limiting the scope of but rather to provide examples of the invention as defined by the claims.        